1. Field of the Invention
The invention relates to a magnetic sensor capable of detecting a change in a magnetic field highly sensitively.
2. Description of the Related Art
In general, when detecting a minute control current flowing in a circuit of a control device accurately, a method is used, where resistors are connected in series in the circuit and a voltage drop of the resistors is measured. However, this may cause some adverse effect on a control system, since a load different from that of the control system is applied. Thus, a method which performs indirect measurement by detecting a gradient of a current magnetic field generated by a control current has been used. For example, the indirect measurement method is achieved by winding a measurement line around a toroidal core, and supplying a control current to the measurement line, to detect a magnetic flux generated in a central part of the toroidal core with a Hall element.
It has been pointed out however, that a current sensor which achieves the method described above has disadvantages, in that a reduction in size is difficult, and that such a current sensor is insufficient in terms of a linearity or a high-frequency response property, and so forth. To address these issues, a magnetic sensor has been proposed, in which a giant magnetoresistive element (which may be hereinafter referred to as a “GMR element”) exhibiting a giant magnetoresistive effect is disposed in an induction magnetic field generated by a control current, and a gradient of the induction magnetic field is detected, as disclosed in U.S. Pat. No. 5,621,377, for example. Also, in this connection, a technology which utilizes a magnetic sensor provided with a GMR element to detect a flaw on a surface of a metal surface, for example, is known. The magnetic sensor utilizing the GMR element makes it possible to relatively improve a detection sensitivity and a response property, and to obtain detection characteristics which are stable even in a temperature variation.
Also, a magnetic sensor utilizing a magnetic tunnel junction element (which may be hereinafter referred to as a “MTJ element”), which exhibits a MR ratio higher than that of the GMR element, has been also under development, as disclosed in Japanese Unexamined Patent Application Publication No, H11-112054 and No. 2001-102659, for example.